A solar cell 10 of the single light-receiving surface type as shown in FIG. 1 is conventionally known to be a crystalline silicon solar cell.
This solar cell 10 is provided with an n-Si layer 16 that is formed by forming a p-n junction on the side of the light-receiving surface of a p-Si layer (p-type crystalline silicon) 18 of a silicon substrate (Si wafer) 11, and an anti-reflection film 14, which is composed of titanium oxide or silicon nitride formed by CVD and the like, and a front surface electrode (light-receiving surface electrode) 12, which is typically composed of silver (Ag) and formed by screen-printing a paste-like composition consisting mainly of Ag (Ag paste) followed by baking, are provided on the front surface thereof. On the other hand, a back surface external connection electrode 22, which is composed of Ag and formed by screen-printing and baking an Ag paste in the same manner as the front surface electrode 12, and an aluminum electrode 20, which demonstrates so-called back surface field (BSF) effects, are provided on the back surface of the p-Si layer 18.
This aluminum electrode 20 is formed over roughly the entire back surface by printing and baking a composition prepared in the form of a paste (including a slurry) consisting mainly of aluminum powder (to be simply referred to as an “aluminum paste”). An Al—Si alloy layer not shown is formed during baking, and a p+ layer 24 is formed as a result of the aluminum being dispersed in the p-Si layer 18. As a result of forming this p+ layer 24, or BSF layer, photo-generated carriers are prevented from recombining in the vicinity of the back surface electrode, thereby realizing improvements in short-circuit current or open-circuit voltage (Voc), for example.
However, it is necessary to form the aluminum electrode 20 as a somewhat thick film (for example, 50 to 60 μm) in order to effectively realize the above-mentioned BSF effects. On the other hand, the silicon substrate (silicon wafer) 11, or in other words the solar cell itself, is required to be even thinner than in the past for reasons such as reducing solar cell production costs and making solar cell modules more compact.
However, reducing the thickness of the substrate (silicon wafer) 11 increases the likelihood of the occurrence of warping or bending and the like in the silicon substrate (wafer) itself when baking to form the aluminum electrode 20 due to differences between the coefficient of thermal expansion of the substrate 11 itself and the coefficient of thermal expansion of the aluminum electrode 20. Consequently, various contrivances have been made in the past to prevent the occurrence of this warping and other deformation.
For example, the following Patent Document 1 discloses a paste-like composition (aluminum paste) for forming an aluminum electrode to which is added an aluminum-containing organic compound. In addition, Patent Document 2 discloses an aluminum paste for forming an aluminum electrode that contains an inorganic compound powder that has a coefficient of thermal expansion that is smaller than that of aluminum and has a melting temperature, softening temperature and decomposition temperature that are all higher than those of aluminum. It is described in these patent documents that, by using the aluminum pastes having the characteristics described above, even if the film thickness of the aluminum electrode is thinner than in the past, adequate BSF effects are obtained, and also that by reducing the film thickness of the aluminum electrode, warping and other deformation that occurs in the wafer during baking can be reduced.
Patent Document 1: Japanese Patent Application Laid-open No. 2000-90734
Patent Document 2: Japanese Patent Application Laid-open No. 2003-223813